Traditionally, energy collectors such as solar air collectors, solar water collectors, and combinations thereof have been connected to the domestic hot water system, since such energy collectors at the northern hemisphere are most efficient during summer time where the solar radiation and the outdoor temperature are high. At the same time the need for heating of the buildings is very low and sometimes non-existing. During periods with lower solar radiation and/or lower outdoor temperature, the efficiency of energy collectors are lower and their ability to collect an amount of energy which is significant compared to the amount of energy which is needed for production of domestic hot water and/or heating of the building is decreased.